Construction of 2D interwoven and 3D metal–organic frameworks (MOFs) of Cd(II): the effect of ancillary ligands on the structure and the catalytic performance for the Knoevenagel reaction

Abstract

Three new Cd(II) metal–organic networks, [{Cd(muco)(bpa)_1.5}·H₂O] (1), [{Cd(muco)(bpee)_1.5}·7H₂O] (2) and [Cd(muco)(4bpdh)·(H₂O)] (3) (where, muco = trans, trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene and 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) have been constructed using mixed ligand systems at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Compounds 1 and 2 are isostructural featuring a 3D framework structure with a 5-connected, {6⁶} net topology. Whereas, compound 3 possess an interesting 3-fold interwoven 2D network with a 4-connected, {4⁴,6²}-sql net topology. Photoluminescence measurements revealed emissions from all the three compounds owing to ligand based charge transfer (n → π* and π → π*) transitions. Catalytic investigations of the compounds for the Knoevenagel reaction unveiled the higher catalytic activity of 3 compared to that of 1 and 2. The higher catalytic performance of 3 has been attributed due to the presence of the basic azine-functionalized pore surface. Remarkably, the catalyst can be facilely separated from the reaction mixture and could be reused without significant degradation in the catalytic activity for five cycles. Compound 3 is a rare example of a 3-fold interwoven 2D network acting as an efficient recyclable heterogeneous catalyst for the Knoevenagel reaction.

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Introduction

Porous coordination polymers (PCPs) or metal–organic frameworks (MOFs) have attracted a great deal of attention in the past two decades due to their intriguing network topology and novel functionality.^1–4 The ability to control the assembly of inorganic building units with organic linkers is beneficial for the controlled design of framework structures which are suitable for uses in hydrogen storage,^5–9 carbon dioxide capture,^10–13 separation,^14–16 catalysis,^17–21 sensing,^22–25 magnetism^26–32 and so on. It has been observed that, the shape and connectivity of organic ligand/linkers play important roles in directing the structure, dimensionality and the topology of the resulting frameworks. MOFs based on only carboxylate ligands were found to be fairly rigid and introduction of bipyridyl linkers in addition to carboxylates imparts flexibility to the structure.^33–38 Thus the bipyridyl spacers not only pillar the metal–carboxylate motifs into higher dimensionality but also support diverse structural topologies.^39–44 The possibility to tailor the length and chemical environment of pyridyl linkers enables construction of functional MOFs for desired applications. In this context efforts have been made in development of porous MOFs with pore surfaces decorated with basic functional groups as efficient heterogeneous catalysts for Knoevenagel condensation.^45–50 The Knoevenagel condensation of aldehydes with compounds containing activated methylene groups is one of the most useful and widely employed methods for carbon–carbon bond formation having potential applications in the field of fine chemicals synthesis.^51,52 Conventionally, this condensation is catalyzed by solid bases like alkali or alkaline-earth metal oxides,⁵³ also by weak bases like primary, secondary and tertiary amines under homogeneous conditions which could require upto 40 mol% catalyst along with the limitation of catalyst recycling. Further, a wide range of catalysts such as Lewis acids,⁵⁴ amine-functionalized solid supports,^55,56 cation-exchanged zeolites,⁵⁷ ionic liquids⁵⁸ and organometallic catalysts⁵⁹ have been employed for this reaction. However, most of these methods possess limitations of using hazardous and carcinogenic solvents, high catalyst loading and non-recoverable catalysts. Therefore, there is a great need for development of new catalytic systems that do not have these problems.

Recently, Morsali and coworkers have demonstrated for the first time the use of MOFs containing azine-functionalized pore surfaces as efficient heterogeneous catalysts for Knoevenagel condensation of aromatic aldehyde and malononitrile.⁶⁰ The presence of basic azine-functionalized pores were found be responsible for enhancing the catalytic performance of the MOF. In the light of these observations, we have constructed three Cd(II) metal–organic compounds using a rigid dicarboxylate ligand and bipyridyl spacers with and without containing azine-functional group, [{Cd(muco)(bpa)_1.5}·H₂O] (1), [{Cd(muco)(bpee)_1.5}·7H₂O] (2) and [Cd(muco)(4bpdh)·(H₂O)] (3) (where, muco = trans, trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene and 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Compounds 1 and 2 are isostructural featuring 3D framework structure with 5-connected, {6⁶} net topology. Whereas, compound 3 possesses an interesting 3-fold interwoven 2D network with 4-connected, {4⁴,6²}-sql net topology. Photoluminescence measurements show emissions from all the three compounds owing to ligand based charge transfer (n → π* and π → π*) transitions. Interestingly, catalytic investigations of 1, 2 and 3 for the Knoevenagel condensation of benzaldehyde with malononitrile showed higher catalytic activity of 3 compared to that of 1 and 2. The higher catalytic performance of 3 has been attributed due to the presence of basic azine-functionalized pore surface. Remarkably, the catalyst can be recycled and reused without significant degradation in the catalytic activity for five cycles. Compound 3 represents a rare example of a 3-fold interwoven 2D coordination network exhibiting an efficient catalytic activity for Knoevenagel reaction. Thus herein we have demonstrated the influence of ancillary ligands on the structure and catalytic properties of the resulting networks. This study demonstrates the importance of basic azine-functionalized pore surface for enhanced catalytic performance for Knoevenagel condensation.

Experimental

Materials

All the reagents employed were commercially available and used as provided without further purification. Cd(OAc)₂·2H₂O, trans, trans-muconic acid (muco), 1,2-bis(4-pyridyl)ethane (bpa), 1,2-bis(4-pyridyl)ethylene (bpee) were obtained from Sigma Aldrich chemical Co. 2,5-Bis(4-pyridyl)-3,4-diaza-2,4-hexadiene was synthesized following previously reported procedure.⁶¹

Physical measurements

Elemental analyses of C, H, N were carried out using a Thermo Fischer Flash 2000 Elemental Analyzer. IR spectra were recorded on a Thermo ScientificNicolet iS10 FT-IR Spectrometer in the region 400–4000 cm⁻¹. Thermogravimetric analyses (TGA) were carried out using Metler Toledo Thermogravimetric analyzer in nitrogen atmosphere (flow rate = 50 mL min⁻¹) in the temperature range of 30–550 °C (heating rate = 10 °C min⁻¹). Powder XRD pattern of the compounds were recorded by using Cu Kα radiation (λ = 1.542 Å; 40 kV, 20 mA) using PANalytical's X’PERT PRO diffractometer. Photoluminescence spectra of the samples were recorded on a Perkin-Elmer LS 55 spectrofluorometer.

1. Synthesis of [{Cd(muco)(bpa)_1.5}·H₂O] (1). Stock solutions of Cd(OAc)₂·2H₂O (0.067 g, 0.25 mmol) in 25 mL of H₂O, H₂muco (0.036 g, 0.25 mmol) neutralized with NaOH (0.020 g, 0.5 mmol) in 12.5 mL of H₂O and bpa (0.046 g, 0.25 mmol) in 12.5 mL of ethanol were prepared. The bpa and the H₂muco solutions were mixed together and stirred for 30 min. Then 2 mL of this solution was slowly and carefully layered over 2 mL of metal salt solution using 1 mL of 2 : 1 (v/v) buffer solution of H₂O/ethanol. The colorless block crystals of [{Cd(muco)(bpa_1.5)}·H₂O] (1) were obtained after eight weeks. Isolated yield: ∼84% based on Cd(II) ion. Phase purity of the compound was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal X-ray data (Fig. S1†). Anal. calcd for C₂₄H₂₄N₃O₅Cd: C, 52.71; H, 4.39; N, 7.68. Found: C, 52.91; H, 4.29; N, 7.71. IR (KBr, cm⁻¹): ν(H₂O), 3440(w); ν(CH-Ar), 3050(w); ν(C=O), 1607(s); ν(C=C-Ar), 1424–1375(s); ν(C–O), 1218(w); ν(=C–H), 832–743(s).

2. Synthesis of [{Cd(muco)(bpee)_1.5}·7H₂O] (2). Compound 2 was synthesized following similar procedure to that of 1 using bpee (0.045 g, 0.25 mmol) instead of bpa. The light-yellowish block crystals of [{Cd(muco)(bpee)_1.5}₂·7H₂O] (2) were obtained after four weeks. Isolated yield: ∼83% based on Cd(II) ion. Phase purity of compound 2 was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal X-ray data (Fig. S2†). Anal. calcd for C₂₄H₃₃N₃O₁₁Cd: C, 44.21; H, 5.11; N, 6.44. Found: C, 44.32; H, 5.19; N, 6.51. IR (KBr, cm⁻¹): ν(H₂O), 3540–3100(bw); ν(C=O), 1607(s); ν(C=C), 1547(s); ν(C=C-Ar), 1424–1375(s); ν(C–O), 1218(w); ν(=C–H), 832–743(s).

3. Synthesis of [Cd(muco)(4bpdh)·(H₂O)] (3). Compound 3 was synthesized following the procedure similar to that of 1 using 4bpdh (0.060 g, 0.25 mmol) in the place of bpa. The yellow colored block crystals of [Cd(muco)(4bpdh)·(H₂O)] (3) were obtained after ten weeks. Isolated yield: ∼78% based on Cd(II) ion. Phase purity of compound 3 was confirmed by comparing the PXRD patterns of the as-synthesized sample and a simulated one from the single crystal X-ray data (Fig. S3†). Anal. calcd for C₂₀H₂₀N₄O₅Cd: C, 47.39; H, 4.37; N, 11.06. Found: C, 46.95; H, 4.60; N, 10.94. IR (KBr, cm⁻¹): ν(H₂O), 3540–3200(bw); ν(CH-Ar), 3050(w); ν(C=N), 1640(s); ν(C=C), 1555–1480(s); ν(C–O), 1350(s).

4. Experimental procedure for the Knoevenagel condensation catalyzed by 1–3. The catalytic reactions were carried out in screw cap glass vials with magnetic stirring and no special precautions were taken to exclude water or air from the reaction vial. The glass vial was loaded with catalyst (2 mol%) in MeOH (3 mL) and the malononitrile (1.2 mmol) and the resulting solution was stirred and then aldehyde (1 mmol) was added. The reaction mixture was kept at r.t. for the indicated time (Table 2), then the progress of the reaction was monitored by GC-MS by taking aliquots at regular intervals of time after extraction in water and ethyl acetate. On completion of the reaction, the catalyst was recovered by centrifugation and washed repeatedly with MeOH and water dried and reused.

X-ray crystallography

Single crystal X-ray structural data of all the three compounds were collected on a CMOS based Bruker D8 Venture PHOTON 100 diffractometer equipped with a INCOATEC micro-focus source with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) operating at 50 kV and 30 mA. The SAINT⁶² program was used for integration of diffraction profiles and absorption correction was made with SADABS program.⁶³ The structures were solved by SIR 92 ⁶⁴ and refined by full matrix least square method using SHELXL-2013 ⁶⁵ and WinGX system, Ver 2013.3.⁶⁶ The non hydrogen atoms in all the structures were located from the difference Fourier map and refined anisotropically. The disordered guest water molecules in compounds 1 and 2 were treated with SQUEEZE option of PLATON⁶⁷ multipurpose software. However, the number of guest water molecules in 1 and 2 were determined from TGA and elemental analyses. All the hydrogen atoms were fixed by HFIX and placed in ideal positions and included in the refinement process using riding model with isotropic thermal parameters. The potential solvent accessible area or void space was calculated using the PLATON⁶⁷ software. All the crystallographic and structure refinement data of the compounds, 1–3 are summarized in Table 1. Selected bond lengths and angles are given in Tables S1–S3,† respectively. Selected hydrogen bond details of the compounds are summarized in Tables S4–S6.† The crystallographic information files are deposited with the CCDC numbers 1444466, 1444467 and 1027847 for compounds 1, 2 and 3 respectively.

Table 1 Crystal data and structure refinement parameters of compounds 1–3

Parameters	1	2	3
a R1 = ∑||Fo| − |Fc||/∑|Fo|.b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Formula	C24H24N3O5Cd	C24H33N3O11Cd	C20H20N4O5Cd
Formula weight	546.92	652.01	508.80
Crystal system	Orthorhombic	Orthorhombic	Monoclinic
Space group	Pbcn (no. 60)	Pbcn (no. 60)	C2/c (no. 15)
a/Å	14.2940(5)	14.094(5)	14.7731(11)
b/Å	16.7484(6)	17.248(5)	18.9394(14)
c/Å	22.1104(7)	22.357(5)	8.7562(6)
α (°)	90	90	90.0
β (°)	90	90	122.551(4)
γ (°)	90	90	90.0
V (Å^3)	5293.3(3)	5435(3)	2065.1(3)
Z	8	8	4
ρ (g cm^−3)	1.327	1.285	1.630
μ (mm^−1)	0.855	0.833	1.097
F (000)	2136	2112	1024
T (K)	298	298	298
λ(Mo Kα) (Å)	0.71073	0.71073	0.71073
Θmin (°)	2.3	2.3	2.6
Θmax (°)	28.4	28.4	28.4
Total data	74 910	69 538	11 876
Unique data	6597	6724	2529
Rint	0.081	0.115	0.137
Data [I > 2σ(I)]	4934	4805	1528
R1^a	0.0370	0.0634	0.0733
wR2^b	0.0866	0.1431	0.1442
S	1.02	1.10	1.04

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Results and discussion

Compounds 1–3 were synthesized by room temperature layering of aqueous solution of Cd(OAc)₂·2H₂O with ethanolic solution of N,N′-donors, bpa, bpee and 4bpdh respectively (Scheme 1).

Scheme 1 General scheme for the synthesis of compounds 1–3.

Crystal structure of [{Cd(muco)(bpa)_1.5}·H₂O] (1) and [{Cd(muco)(bpee)_1.5}·7H₂O] (2)

Both 1 and 2 are isostructural and crystallize in the orthorhombic crystal system with the Pbcn space group. X-ray structure determination reveals a 3D framework constituted by a bridging muconate and the bipyridyl (bpa and bpee) spacers. In both the compounds, hepta-coordinated Cd(II) ions are in a distorted pentagonal bipyramidal geometry with CdO₄N₃ chromophore satisfied by four oxygen (O1, O2, O3 and O4) atoms from two chelated carboxylate groups of muconate and three 4-pyridyl nitrogen (N1, N2 and N3) atoms of one and half molecules of bpee/bpa linker (Fig. 1a and 2a). The Cd1–O and Cd1–N bond lengths in 1(2) are in the range 2.308(2)–2.562(2) Å (2.336(5)–2.536(4) Å) and 2.337(2)–2.376(2) Å (2.326(5)–2.355(5) Å), respectively (Tables S1 and S2†). The muconate ligand connects the Cd(II) ions through chelating bidentated μ₂-O fashion forming [Cd-muco-Cd]_n 1D chain along the crystallographic c-axis which are stacked in –AB–AB– fashion (Fig. 1 and 2). These 1D chains are further connected by bpa/bpee linkers in two different directions to generate a 3D framework (Fig. 1c and 2d). If one assumes carboxylate coordinations to Cd(II) ion as monodentate then the hepta-coordinated Cd(II) nodes can be approximated to penta-coordination. In fact, topological analyses of both the compounds by TOPOS⁶⁸ suggests that each Cd(II) ion acts as 5-connected node with Schläfli point symbol {6¹⁰} and the overall structure has a non-interpenetrating 3D framework (Fig. 1d and 2e). The solvent accessible void volume of 1 is ∼21.7% (1147.8 ų) and of 2 is ∼25.9% (1406.2 ų) per unit cell calculated from PLATON⁶⁷ software. The distance between the two adjacent Cd⋯Cd centers in 1(2) along Cd⋯muco⋯Cd due to bridging muco is 11.499 (11.616) Å and along Cd⋯bpa(bpee)⋯Cd is 13.983 (13.874) Å, respectively.

Fig. 1 (a) The coordination environment around Cd(II) in 1: the hydrogen atoms and solvent molecules are omitted for clarity. (b) View of [Cd-muco-Cd]_n 1D chains along the crystallographic c-axis stacked in AB–AB fashion. (c) View of 3D pillar-layered framework and (d) shows the topological representation of the 3D framework.

Fig. 2 (a) The coordination environment around Cd(II) in 2: the hydrogen atoms and solvent molecules are omitted for clarity. (b) View of [Cd-muco-Cd]_n 1D chains along the crystallographic c-axis. (c) Stacking of 1D chains in AB–AB fashion. (d) View of 3D pillar-layered framework and (e) shows the topological representation of the 3D framework.

Crystal structure of [Cd(muco)(4bpdh)·(H₂O)] (3)

Compound 3 crystallizes in the monoclinic crystal system with the C2/c space group. X-ray structure determination reveals a 2D network structure constituted by a bridging muconate and a 4bpdh spacer. The asymmetric unit consists of a Cd(II), muconate and a 4bpdh spacer including a coordinated water molecule (Fig. 3a). The hepta-coordinated Cd(II) ion is in a distorted pentagonal bipyramidal geometry with CdO₅N₂ chromophore satisfied by four carboxylate oxygen (O1, O2, O1a and O2a, where a = 2 − x, y, 5/2 − z) atoms of a bridging muconate and an oxygen (O1w) atom of a coordinated water molecule occupying the equatorial positions, the axial positions are occupied by two 4-pyridyl nitrogen (N1 and N1a, where a = 2 − x, y, 5/2 − z) atoms of a 4bpdh spacer (Fig. 3a). The Cd–O bond lengths are in the range 2.331–2.436 Å and the Cd–N bond length is 2.365 Å (Table S3†). Here the muconate ligand bridges two Cd(II) ions through a chelating bidented μ₂-O fashion forming a [Cd-muco-Cd]_n 1D zig-zag polymeric chains which are further connected by 4bpdh spacers forming a 2D corrugated sheet which houses a large rectangular channels of dimension ∼7.80 × 12.60 Ų (Fig. 3b). The presence of large rectangular channels along with the zig-zag arrangement of 1D chains facilitates interweaving of two other 2D nets in to the void space to generate a novel 3-fold interwoven 2D network (Fig. 3c and d). Topological analysis by TOPOS⁶⁸ suggests that each Cd(II) ion acts as a 4-connecting node and the overall structure has 3-fold interwoven 2D network with Schläfli point symbol {4⁴,6²} (Fig. 3e). The distance between the two adjacent Cd1⋯Cd1 centers along Cd⋯muco⋯Cd due to bridging muconate is 11.695 Å and along Cd⋯4bpdh⋯Cd is 15.701 Å.

Fig. 3 (a) The coordination environment around Cd(II) ion in 3: the hydrogen atoms are omitted for clarity (symmetry codes for O1a, O2a, N1a and N2a, where a = −x, y, 1/2 − z) (b) view of a single 2D network constructed by Cd(II), muco, and 4bpdh spacer. (c) 3-Fold interwoven 2D network (three different 2D nets are shown in three different colors) (d) shows CPK view of the 3-fold interwoven 2D network of 3. (e) Topological representation of the 3-fold interwoven nets in 3 (three different 2D nets are shown in three different colors).

Effect of ancillary ligand on the structures of compounds 1–3

All the three compounds were constructed by room temperature reaction of Cd(II) ion, muconate dianion and N,N′-donor (bpa, bpee and 4bpdh) spacers, respectively. The 3D framework in 1 and 2 is formed by bridging of 1D [Cd-muco-Cd]_n polymeric chains by bpa and bpee spacers (Fig. 1 and 2). Whereas, the use of a relatively longer spacer, 4bpdh results in a 2D microporous network with large pore size of 7.80 × 12.60 Ų. The availability of large void size along with the zig-zag arrangement of 1D chains in 3 facilitates interweaving of two other 2D nets in to the empty space to generate a novel 3-fold interwoven 2D network (Fig. 3c and d). Thus the nature of ancillary ligand plays an important role in directing the structure and topology of the resulting framework.

Photoluminescence properties of compounds 1–3

Luminescent MOFs containing d¹⁰ metal ions and organic chromophores are attracting much interest due to their potential applications in the development of chemosensors, electroluminescence displays and so on.^69–72 In this regard, the room temperature photoluminescence properties of the compounds 1–3 and free muconic acid were investigated in the solution state using dispersed samples in dimethylformamide (DMF). Compounds 1, 2 and 3 exhibit intense absorption bands around 266, 288 and 267 nm, respectively (Fig. S4†). Further, the emission bands with maxima centered around 420 nm (λ_ex = 270 nm) were observed for all the three compounds and in the same peak position free muconate ligand also shows emission (Fig. 4). Hence, the emissions from compounds 1–3 can be ascribed due to ligand based charge transfer (n–π* and π–π*) transitions.^73,74

Fig. 4 Room temperature photoluminescence spectra of free muconic acid and compounds 1–3 dispersed in DMF (λ_ex = 270 nm).

Knoevenagel condensation

Compounds 1–3 were assessed for their activity in Knoevenagel condensation of aromatic aldehydes with active methylene compounds (Scheme 2). Motivated by green chemistry principles the reaction was carried out with 2 mol% of compounds at room temperature in methanol solvent. The test reaction of Knoevenagel condensation of benzaldehyde with malononitrile resulted the condensation product with about 41%, 39% and 100% yield of 2-benzylidene malononitrile after 3 h catalyzed by compound 1, 2 and 3 respectively (Fig. 5). Furthermore, the blank reaction between benzaldehyde and malononitrile carried out in the absence of catalyst 1–3 results in almost negligible yield (14%) of condensation product, suggesting the requirement of a catalyst (Fig. 5). The higher catalytic performance of 3 compared to that of 1 and 2 can be attributed due to the presence of basic azine-functionalized pore surface. This high catalytic activity of 3 for Knoevenagel condensation encouraged us to extend the catalytic study to various other substituted benzaldehydes. The progress of the catalytic reaction was monitored by taking aliquots at regular time intervals and analyzed by GC. Further, introduction of 2 mol% of compound 3 as catalyst results in the condensation product with complete conversion of benzaldehyde after 90 min (Fig. 5, entry 1, Table 2) indicating the excellent catalytic performance of 3 for the Knoevenagel condensation. Furthermore, no additional byproducts were observed during the reaction, so the selectivity for 2-benzylidene malononitrile is 100%. The catalytic activity of compound 3 for the Knoevenagel condensation of benzaldehyde and malononitrile is comparable to the recent reports of MOF based catalysts (Table 3).

Scheme 2 Knoevenagel condensation reaction of aromatic aldehydes with malononitrile and ethyl cyanoacetate catalyzed by compound 3.

Fig. 5 Knoevenagel condensation of benzaldehyde and malononitrile carried out without and with the addition of compounds 1–3.

Table 2 Reaction of various aromatic aldehydes with malononitrile and ethyl cyanoacetate catalyzed by compound 3

Entry no.	R1	R2,R3	Time [min]	Temperature [°C]	Conversion^a [%]
a Reaction conditions: aldehyde (1 mmol), malononitrile (1.2 mmol), solvent: methanol (3 mL), catalyst (2 mol%), 3 h stirring. Yield calculated by GCMS on the basis of aromatic benzaldehydes.
1	–H	R2,R3 = –CN	90	r.t.	100
2	p-NO2	R2,R3 = –CN	30	r.t.	100
3	p-Cl	R2,R3 = –CN	60	r.t.	100
4	p-OMe	R2,R3 = –CN	180	r.t.	96.2
5	p-Me	R2,R3 = –CN	180	r.t.	99.3
6	–H	R2 = –CN, R3 = –COOEt	180	r.t.	97.8
7	p-NO2	R2 = –CN, R3 = –COOEt	120	r.t.	99.1
8	p-Cl	R2 = –CN, R3 = –COOEt	150	r.t.	99.2

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Table 3 Comparison of the catalytic activity of various MOFs in the Knoevenagel reaction of benzaldehyde and malononitrile

Sr. no.	Catalyst	Mol (%)	Time	Solvent	Temp [°C]	Yield (%)	Ref.
1	[Cd(4-btapa)2(NO3)2]·6H2O·2DMF	5	12 h	C6H6	r.t.	98	75
2	[Cd(bipd)2(DMF)2]·(ClO4)2·(2DMF)	4	30 min	C6H6	r.t.	93	59
3	Pb(cpna)2·2DMF·6H2O	3	24 h	CH3CN	r.t.	100	45
4	ZIF-8	5	3 h	Toluene	r.t.	100	46
5	ZIF-9	5	4 h	Toluene	r.t.	99	76
6	[Gd2(tnbd)3(DMF)4]·4DMF·3H2O	10	20 min	C6H6	r.t.	96	77
7	[Zn(oba)(4-bpdh)0.5]n·(DMF)y	2	30 min	H2O	r.t.	100	78
8	Compound 3	2	90 min	MeOH	r.t	100	This work

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Further, to rule out the possibility of homogeneous catalysis due to some of the active sites (4bpdh linker) leached from the framework, the catalyst was removed by centrifugation after 15 min and the reaction solution was allowed to stir for additional 2.5 h at room temperature with aliquots being sampled at different time intervals and analyzed by GC. The GC analysis revealed that there was no further reaction observed suggesting no contribution from leached species and the catalytic conversion was only being possible in the presence of the compound 3 confirming the heterogeneous nature of the reaction (Fig. 6).

Fig. 6 Leaching test confirming no contribution from homogeneous catalysis of active species leaching into reaction solution.

Having confirmed the catalytic activity of 3 for Knoevenagel condensation between benzaldehyde and malononitrile we extended the study to various substituted benzaldehydes and active methylene compounds and the results are listed in Table 2. It is clear from the Table 2 that the catalytic activity of compound 3 for 4-nitrobenzaldehyde is much faster than that of benzaldehyde as the reaction for the former can reach to 100% completion within 30 min, whereas for the later it takes 90 min for 100% conversion (Fig. 7, entry 1 and 2 in Table 2). Furthermore, the reaction of 4-chlorobenzaldehyde proceeded to give 100% conversion in 60 min with the catalytic rate lies in between that of benzaldehyde and 4-nitrobenzaldehyde (Fig. 7, entry 3 in Table 2). Thus the reactivity trend unveils the strong accelerating influence of electron-withdrawing nitro group as expected for a reaction involving nucleophilic attack at the carbonyl group. Stronger the electron-withdrawing ability of the substituent, faster the activation of aldehyde for nucleophilic attack at the carbonyl group and hence higher the conversion. On the other hand, the reactions of electron-donating para-substituted benzaldehydes (Fig. 7, entry 4 and 5 in Table 2) proceeded to give ≥96% conversion after 120 min. Though the conversions are similar to that of non-substituted benzaldehyde but the initial rates of p-methyl and p-methoxy benzaldehyde are found to be slower than that of benzaldehyde as shown in Fig. 7.

Fig. 7 Effect of different substituents on the catalytic conversion of Knoevenagel reaction catalyzed by compound 3.

The catalytic study was further extended to the Knoevenagel condensation of benzaldehyde and its substituted derivatives with ethyl cyanoacetate under similar reaction conditions. The results show that (entry 6, Table 2) ethyl cyanoacetate led to ≥97% conversion of benzaldehyde after 3 h, whereas in case of malononitrile the conversion was close to 100% within 90 min. The reactivity difference of the methylene compounds follow with their acidity scale, which increases in the order ethyl cyanoacetate (pK_a = 13.1)⁷⁹ < malononitrile (pK_a = 11.1).⁸⁰ Further reaction of p-nitro and p-chlorobenzaldehyde with ethyl cyanoacetate follows similar trend as that of malononitrile with the rate higher for the former compared to that of later (entries 7 and 8 in Table 2).

To determine the recyclability of the catalyst, the catalyst was isolated after the reaction by filtration and washed with methanol and dried in vacuum, the powder XRD pattern of the recycled sample of 3 matches well with that of parent compound suggesting retaining of the original framework structure (Fig. S5†). The catalyst was recycled and reused for five cycles without significant degradation in the original catalytic activity and the structural rigidity (Fig. 8).

Fig. 8 Catalyst recycling test.

Mechanism for Knovenagel condensation catalyzed by 1–3

The Knoevenagel condensation reactions of aldehydes with compounds containing active methylene groups are generally catalyzed by several bases like alkali or alkaline-earth metal oxides, amines, etc. Here in compounds 1 and 2 the intrinsic basicity arises due to the presence of carboxylate oxygens of muconate ligand (Fig. 9).⁸¹ The metal coordinated carboxylate oxygens which are projecting towards pore can promote Knoevenagel condensation of benzaldehyde and malononitrile to yield the condensation product with up to 40% yield (Fig. 5). On the other hand, in compound 3 in addition to metal carboxylate oxygen sites the presence of more basic azine groups in the pore surfaces contribute for the enhanced basicity of the framework resulting in higher catalytic activity in comparison to that of 1 and 2. Here, the electron lone-pairs on adjacent nitrogen atoms of azine groups destabilize each other by electronic repulsion and thereby increase the basicity of the 4bpdh linker and the framework. Hence, the higher catalytic activity of compound 3 is predominantly due to the availability of azine basic sites of 4bpdh linkers which is also supported by the observation of relatively high catalytic activity of 4bpdh linker in comparison to that of compounds 1 and 2 (Fig. 5). The proposed mechanism of Knoevenagel condensation reaction catalyzed by compound 3 due to the azine group of 4bpdh linker has been shown in Scheme 3.

Fig. 9 View showing the presence of potential basic sites in the framework 1 and 2.

Scheme 3 Proposed reaction mechanism for Knoevenagel condensation by compound 3 (only the azine part of the 4bpdh linker has been shown for compound 3).

Thermal stability of the compounds 1–3

Compound 1 shows a weight loss of ∼1.45% around 100 °C corresponding to the loss of one guest water molecule (calc. wt% 1.4) and the dehydrated framework is stable up to 210 °C. In the temperature range of 210–400 °C a second weight loss of ∼51.9% was observed which corresponds to the loss of 1.5 molecules of bpa linker (calc. wt% 52.26) (Fig. 10). Compound 2 shows a weight loss of ∼19.72% from 60–210 °C which corresponds to the loss of seven guest water molecules (calc. wt% 19.32). The second weight loss of ∼51.10% in temperature range 210–405 °C corresponds to the loss of one bpee and one muconate molecule (calc. wt% 49.78) (Fig. 10). Compound 3 shows a weight loss of ∼3.42% in the temperature range 80–160 °C which corresponds to the loss of one coordinated water molecule (calc. wt% 3.55). The second weight loss of ∼48.19% in the temperature range 250–385 °C corresponds to the loss of the 4bpdh linker (calc. wt% 47.41) (Fig. 10).

Fig. 10 TGA plot for the compounds 1–3.

Conclusions

Herein, we have successfully constructed three new Cd(II)–organic networks using mixed ligand systems by room temperature self-assembly and characterized them structurally. The use of bpa and bpee as spacers results in two isostructural co-ordination polymers featuring 3D framework structure with 5-connected, {6⁶} net topology. Whereas, the use of a longer spacer, 4bpdh results in an interesting 3-fold interwoven 2D network with 4-connected, {4⁴,6²}-sql net topology. Thus, the influence of N,N′-donor spacers in directing the crystal structure and the topology of the resulting MOF has been demonstrated. Photoluminescence study of the compounds carried out at room temperature showed emissions due to ligand based charge transfer (n → π* and π → π*) transitions. Catalytic investigations of the compounds for the Knoevenagel reaction revealed higher catalytic activity of 3 compared to those of 1 and 2. The higher catalytic performance of 3 has been attributed due to the presence of basic azine-functionalized pore surface. Compound 3 is a rare example of 3-fold interwoven 2D coordination network exhibiting efficient catalytic activity for Knoevenagel condensation. This study demonstrates the influence of ancillary ligands on the structure and catalytic properties of the resulting co-ordination polymers.

Acknowledgements

Authors gratefully acknowledge the financial support from the Department of Science and Technology (DST), Government of India (Fast Track Proposal).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Powder XRD, crystallographic tables, UV-vis plot. CCDC 1444466, 1444467 and 1027847. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra01647b

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